Correcting Biases In Microseismic-Event Data

Abstract

Microseismic-event data can be corrected (e.g., to reduce or eliminate bias). For example, a first distribution of microseismic events that occurred in a first area of a subterranean formation can be determined. The first distribution can be used as a reference distribution. A second distribution of microseismic events that occurred in a second area of the subterranean formation can also be determined. The second area of the subterranean formation can be farther from an observation well than the first area. The second distribution can be corrected by including, in the second distribution, microseismic events that have characteristics tailored for reducing a difference between the second distribution and the first distribution.

1 . A method comprising:
 generating, by a processing device, a first distribution that is representative of a distribution of microseismic events that occurred in a first area of a subterranean formation;   generating, by the processing device, a second distribution that is representative of another distribution of microseismic events that occurred in a second area of the subterranean formation that is farther from an observation well than the first area; and   correcting, by the processing device, the second distribution by including in the second distribution microseismic events that have characteristics for reducing a difference between the first distribution and the second distribution.   
     
     
         2 . The method of  claim 1 , further comprising generating the first distribution by:
 determining a plurality of distances between primary events and secondary events that occurred within the first area of the subterranean formation, a primary event being a microseismic event for which a characteristic satisfies a condition and a secondary event being another microseismic event for which the characteristic does not satisfy the condition; and   determining how many times each distance is present in the plurality of distances.   
     
     
         3 . The method of  claim 2 , wherein the characteristic is a magnitude and the condition includes exceeding a magnitude threshold, the method further comprising generating the second distribution by:
 determining another plurality of distances between primary events and secondary events that occurred within the second area of the subterranean formation; and   determining how many times each distance is present in the other plurality of distances.   
     
     
         4 . The method of  claim 3 , further comprising:
 receiving, from a sensor positioned in a wellbore, sensor signals associated with microseismic events that occurred in the subterranean formation;   determining microseismic-event data based on the sensor signals; and   for each microseismic event in the microseismic-event data:
 determining that the microseismic event is the primary event based on the characteristic for the microseismic event satisfying the condition; or 
 determining that the microseismic event is the secondary event based on the characteristic for the microseismic event not satisfying the condition. 
   
     
     
         5 . The method of  claim 4 , further comprising:
 categorizing the microseismic events in the microseismic-event data into a plurality of groups based on respective distances of the microseismic events to the observation well;   selecting a particular group of the plurality of groups for use as a reference group based on the particular group having microseismic events that are closer to the observation well than a remainder of the groups in the plurality of groups; and   determining the first distribution using the microseismic events in the reference group.   
     
     
         6 . The method of  claim 5 , further comprising, analyzing each group of the remainder of the groups by:
 generating a respective distribution of the respective microseismic events in a respective group;   comparing the respective distribution to the first distribution to determine if there is an inconsistency between the respective distribution and the first distribution; and   based on determining that there is the inconsistency between the respective distribution and the first distribution:
 generating a corrected version of the respective group by including in the respective group one or more microseismic events having characteristics for reducing the inconsistency between the respective distribution and the first distribution; and 
 generating a corrected version of the respective distribution using the corrected version of the respective group. 
   
     
     
         7 . The method of  claim 6 , further comprising:
 determining that all of the groups on the remainder of the groups have been analyzed;   based on determining that all of the groups in the remainder of the groups have been analyzed, determining that an end criterion has been satisfied; and   based on determining that the end criterion has been satisfied:
 determining an estimated distribution of microseismic events that occurred in the subterranean formation by combining information from a plurality of corrected versions of respective distributions; and 
 displaying a graph indicating the estimated distribution of microseismic events. 
   
     
     
         8 . A system comprising:
 a sensor positionable proximate to a subterranean formation for detecting microseismic events in the subterranean formation and transmitting sensor signals associated with the microseismic events; and   a computing device communicatively coupled to the sensor for:
 generating, based on the sensor signals, a first distribution that is representative of a distribution of microseismic events that occurred in a first area of the subterranean formation; 
 generating, based on the sensor signals, a second distribution that is representative of another distribution of microseismic events that occurred in a second area of the subterranean formation that is farther from the sensor than the first area; 
 generating a corrected version of the second distribution by including in the second distribution microseismic events that have characteristics for reducing a difference between the first distribution and the second distribution; and 
 displaying the corrected version of the second distribution via a display device. 
   
     
     
         9 . The system of  claim 8 , wherein the sensor comprises a geophone and the computing device comprises:
 a processing device; and   a memory device on which instructions executable by the processing device are stored for causing the processing device to generate the first distribution by:
 determining a plurality of distances between primary events and secondary events that occurred within the first area of the subterranean formation, a primary event being a microseismic event for which a characteristic satisfies a condition and a secondary event being another microseismic event for which the characteristic does not satisfy the condition; and 
 determining how many times each distance is present in the plurality of distances. 
   
     
     
         10 . The system of  claim 9 , wherein the characteristic is a magnitude, the condition includes exceeding a magnitude threshold, and the memory device further comprises instructions that are executable by the processing device for causing the processing device to generate the second distribution by:
 determining another plurality of distances between primary events and secondary events that occurred within the second area of the subterranean formation; and   determining how many times each distance is present in the other plurality of distances.   
     
     
         11 . The system of  claim 10 , wherein the memory device further comprises instructions that are executable by the processing device for causing the processing device to:
 determine microseismic-event data based on the sensor signals; and   for each microseismic event in the microseismic-event data:
 determine that the microseismic event is the primary event based on the characteristic for the microseismic event satisfying the condition; or 
 determine that the microseismic event is the secondary event based on the characteristic for the microseismic event not satisfying the condition. 
   
     
     
         12 . The system of  claim 11 , wherein the memory device further comprises instructions that are executable by the processing device for causing the processing device to:
 categorize the microseismic events in the microseismic-event data into a plurality of groups based on respective distances of the microseismic events to the sensor;   select a particular group of the plurality of groups for use as a reference group based on the particular group having microseismic events that are closer to the sensor than a remainder of the groups in the plurality of groups; and   determine the first distribution using the microseismic events in the reference group.   
     
     
         13 . The system of  claim 12 , wherein the memory device further comprises instructions that are executable by the processing device for causing the processing device to analyze each group of the remainder of the groups by:
 generating a respective distribution of the respective microseismic events in a respective group;   comparing the respective distribution to the first distribution to determine if there is an inconsistency between the respective distribution and the first distribution; and   based on determining that there is the inconsistency between the respective distribution and the first distribution:
 generating a corrected version of the respective group by including in the respective group one or more microseismic events having characteristics for reducing the inconsistency between the respective distribution and the first distribution; and 
 generating a corrected version of the respective distribution using the corrected version of the respective group. 
   
     
     
         14 . The system of  claim 13 , wherein the memory device further comprises instructions that are executable by the processing device for causing the processing device to:
 determine whether all of the groups on the remainder of the groups have been analyzed;   based on determining that all of the groups in the remainder of the groups have been analyzed, determine whether an end criterion has been satisfied;   based on determining that the end criterion has been satisfied:
 determine an estimated distribution of microseismic events that occurred in the subterranean formation by combining information from a plurality of corrected versions of respective models; and 
 display, on the display device, a graph indicating the estimated distribution of microseismic events. 
   
     
     
         15 . A non-transitory computer-readable medium that includes instructions that are executable by a processing device for causing the processing device to:
 generate a first distribution that is representative of a distribution of microseismic events that occurred in a first area of a subterranean formation;   generate a second distribution that is representative of another distribution of microseismic events that occurred in a second area of the subterranean formation that is farther from an observation well than the first area; and   correct the second distribution by including in the second distribution microseismic events that have characteristics for reducing a difference between the first distribution and the second distribution.   
     
     
         16 . The non-transitory computer-readable medium of  claim 15 , further comprising instructions that are executable by the processing device for causing the processing device to generate the first distribution by:
 determining a plurality of distances between primary events and secondary events that occurred within the first area of the subterranean formation, a primary event being a microseismic event for which a characteristic satisfies a condition and a secondary event being another microseismic event for which the characteristic does not satisfy the condition; and   determining how many times each distance is present in the plurality of distances.   
     
     
         17 . The non-transitory computer-readable medium of  claim 16 , wherein the characteristic is a magnitude, the criterion includes exceeding a magnitude threshold, and further comprising instructions that are executable by the processing device for causing the processing device to generate the second distribution by:
 determining another plurality of distances between primary events and secondary events that occurred within the second area of the subterranean formation; and   determining how many times each distance is present in the other plurality of distances.   
     
     
         18 . The non-transitory computer-readable medium of  claim 17 , further comprising instructions that are executable by the processing device for causing the processing device to:
 receive, from a sensor positioned in a wellbore, sensor signals associated with microseismic events that occurred in the subterranean formation;   determine microseismic-event data based on the sensor signals; and   for each microseismic event in the microseismic-event data:
 determine that the microseismic event is the primary event based on the characteristic for the microseismic event satisfying the condition; or 
 determine that the microseismic event is the secondary event based on the characteristic for the microseismic event not satisfying the condition. 
   
     
     
         19 . The non-transitory computer-readable medium of  claim 18 , further comprising instructions that are executable by the processing device for causing the processing device to:
 categorize the microseismic events in the microseismic-event data into a plurality of groups based on respective distances of the microseismic events to the observation well;   select a particular group of the plurality of groups for use as a reference group based on the particular group having microseismic events that are closer to the observation well than a remainder of the groups in the plurality of groups; and   determine the first distribution using the microseismic events in the reference group.   
     
     
         20 . The non-transitory computer-readable medium of  claim 19 , further comprising instructions that are executable by the processing device for causing the processing device to analyze each group of the remainder of the groups by:
 generating a respective distribution based on the respective microseismic events in a respective group;   comparing the respective distribution to the first distribution to determine if there is an inconsistency between the respective distribution and the first distribution; and   based on determining that there is the inconsistency between the respective distribution and the first distribution:
 generating a corrected version of the respective group by including in the respective group one or more microseismic events having characteristics for reducing the inconsistency between the respective distribution and the first distribution; and 
 generating a corrected version of the respective distribution using the corrected version of the respective group. 
   
     
     
         21 - 35 . (canceled)